MICROHARDNESS UNDER STRAIN - 4 - REVERSIBLE MICROHARDNESS IN POLYBLOCK THERMOPLASTIC ELASTOMERS WITH POLY(BUTYLENE TEREPHTHALATE) AS HARD SEGMENTS

The microhardness (H) technique was recently applied to poly(butylene terephthalate) (PBT) and its multiblock copolymers for examination of the stress-induced polymorphic transition. Following these investigati ons the present study attempts to observe the reversible variation of microhardness under strain. For this purpose bristles of drawn and ann ealed (at 160 degrees C in vacuum) poly(ether ester) (PEE) were charac terized with respect to their microhardness at various stages of tensi le deformation. H was measured under and after loading (sigma) in defo rmation steps of 5% each. In accordance with previous results on PBT a nd PEE, H values at sigma not equal 0 show a sharp drop (by 40%) in a relatively narrow deformation interval (epsilon = 10-15%), owing to th e stress-induced alpha double left right arrow beta polymorphic transi tion. The hardness measurements at sigma = 0 show a continuous decreas e of H with a remaining strain. H values at sigma = 0, corresponding t o plastic deformation up to 5%, are much higher than the corresponding ones taken under stress at an overall deformation between 10 and 25%. The higher H values are explained by the regeneration of the starting alpha modification. Results reveal that in materials characterized by high and reversible deformability it is possible to observe a reversi ble microhardness behaviour, provided the strain-induced structural ch anges are reversible.